The advantages of monofunctional non-acidic platinum L-zeolite catalysts for catalytic reforming were discovered in the late 1970's. U.S. Pat. No. 4,104,320 to Bernard and Nury discloses that it is possible to dehydrocyclize paraffins to produce aromatics with high selectivity using a monofunctional non-acidic L-zeolite catalyst. This L-zeolite catalyst has exchangeable cations of which at least 90% are sodium, lithium, potassium, rubidium or cesium, and contains at least one Group VIII noble metal or tin or germanium.
Later, Buss and Hughes developed improved L-zeolite catalysts for reforming petroleum naphthas. These catalysts comprise a large pore zeolite exchanged with an alkaline earth metal containing one or more Group VIII metals (preferably platinum). See U.S. Pat. Nos. 4,434,311; 4,435,283; 4,447,316, and 4,517,306. Barium exchanged catalysts were found to provide even higher selectivities than the corresponding alkali exchanged L-zeolite catalysts disclosed in U.S. Pat. No. 4,104,320. The high selectivity catalysts of Bernard and Nury, and of Buss and Hughes, are all "non-acidic" and are referred to herein as "monofunctional catalysts". These catalysts are highly selective for forming aromatics via dehydrocyclization of paraffins.
After more than 10 years of extensive research and engineering, the use of L-zeolite catalysts for reforming was commercialized. This commercialization required many additional discoveries. Two key discoveries were the criticality of ultra-low sulfur levels in the feed, and the impact of these ultra-low sulfur levels on reactor metallurgy, i.e., the discovery of the need to prevent coking, carburization and metal dusting.
While commercialization of ultra-low sulfur reforming was being pursued, a second generation of platinum L-zeolite catalysts was being developed. These new catalysts (referred to herein generically as "hiz-cats") are produced by treating L-zeolites with halogen-containing compounds, such as halocarbons (e.g., U.S. Pat. No. 5,091,351 to Murakawa et al.) or ammonium salts, (e.g., EP 498,182A and U.S. Pat. No. 5,354,933 to Ohashi et al.) and Group VIII metals. It is disclosed that these halided catalysts are useful for preparing aromatic hydrocarbons such as benzene, toluene and xylenes from C.sub.6 -C.sub.8 aliphatic hydrocarbons in high yield. They allow operations at high severity, tolerate a wide range of hydrocarbon feeds, and show high activity and long life in screening studies. Other patents that disclose hiz-cats include U.S. Pat. No. 4,681,865, U.S. Pat. No. 4,761,512 and U.S. Pat. No. 5,073,652 to Katsuno et al.; and U.S. Pat. No. 5,196,631 and U.S. Pat. No. 5,260,238 to Murakawa et al.
Yet, our attempts to commercialize one of these hiz-cats resulted in a catalyst with poor performance. When the patented recipe for preparing a bound halided L-zeolite catalysts was followed, we discovered that the resulting catalyst was unexpectedly sensitive to reforming startup conditions. The catalyst performed well in screening studies using high gas velocity, rapid heat rate, "laboratory" startup (LSU) conditions; but surprisingly, its fouling rate was unacceptably high when low velocity, simulated "commercial" startup (SCSU) conditions were employed.
Scale-up and commercialization of a new process, particularly one using a new catalyst, entail numerous changes from laboratory procedures. For instance, limitations due to commercial equipment impact scale-up designs and the way commercial processes are operated. For example, commercial gas compressors set limits on gas flow rates, and heat transfer limitations influence reactor configurations, designs, and operating conditions. Moreover, laboratory studies are generally done to get results in a reasonable time frame, while commercial operations are intended to operate for much longer times. One example of unpredictability in the process scale-up area is Heyse et al., as described in U.S. Pat. No. 5,675,376. Here, potentially catastrophic carburization and metal dusting problems were only first observed when low sulfur reforming was being tested in commercial reforming equipment. The metallurgy, process configuration, and temperature of commercial reforming operations lead to coke plugging problems not observed in laboratory studies.
Commercialization and scale-up also requires working with quantities of materials that are 100 to 1,000,000 times larger than used in laboratory studies. This increased scale typically requires that catalyst preparation procedures be modified. A process as simple as drying is done quite differently in the laboratory than at a commercial plant. Often, these scale differences do not significantly affect catalyst performance, but occasionally they do.
Furthermore, the catalyst composition is often modified to meet the demands of commercial operations, e.g. to attain needed crush strength. Also, (unbound) zeolite crystals or powders are often used in laboratory studies. In contrast, bound zeolites are usually used in commercial operations. This is because bound zeolites reduce the pressure drop through large reactor, provide improved gas and liquid flow rates, and are easier to load and unload. Yet it is known that the performance of bound catalysts, especially bound zeolites, can be quite different from that of unbound powders. Sometimes, the type of binder or binding method affects catalyst performance. Sometimes special steps that are only done for bound catalysts, such as calcination of the bound zeolite, affects catalyst performance.
Thus, studies on powdered zeolites do not always predict the commercial performance of bound zeolites. One example that shows the unpredictability of catalyst performance associated with adding a binder is Mohr et al., in U.S. Pat. No. 5,106,803. Mohr et al., disclose the criticality of the water sensitivity index of a bound L-zeolite catalyst. This property was not observed to be important for the unbound catalyst.
Yet one must assume that scale-up will proceed satisfactorily, since it is not possible to predict what factors will be critical for a particular new process or new catalyst. For halided zeolite catalysts, there is no suggestion in the art that there might be problems associated with preparing bound catalysts or with the use of these catalysts in commercial operations, particularly problems related to catalyst startup procedures. Indeed, there is nothing in the art that teaches the importance of washing or otherwise treating a catalyst base (i.e., a bound zeolite) or a bound zeolite catalyst before adding halide, as described hereinbelow.
Treating conventional, non-halided zeolites--including L-zeolite--with water or an aqueous solution is known. For example, Kao et al., in U.S. Pat. No. 4,987,109 adjust the pH of L-zeolite crystals by washing them so as to provide a zeolite of pH 9.4-10.0. These washed zeolites are then bound. Washing the zeolite crystals to this pH improves catalyst activity. Also, Poeppelmeir et al., in U.S. Pat. No. 4,595,668, teach having the noble metal of in a zeolite dispersed in particles having a diameter of less than 7 angstroms. In Example 6, they describe how to prepare a bound catalyst of their invention using an oxychlorination step. Another example is Poeppelmeir et al., in U.S. Pat. No. 4,568,656. Here a bound L-zeolite is contacted with an aqueous solution containing a Pt salt and a non-platinum salt (e.g., KCl). This mixture is aged to distribute the Pt uniformly (see Example 1.) Also, Buss and Hughes in U.S. Pat. No. 4,721,694 teach preparing platinum Ba L-zeolite catalysts using a barium ion exchange before Pt impregnation. The ion exchange replaces potassium with barium in the catalyst and is taught to improve selectivity. For example, in Col 12, they discuss treating extruded pellets by ion exchange with a barium solution, followed by Pt impregnation. These patents are all incorporated herein by reference.
It is also known to wash powdered zeolite catalysts containing halides. For instance, Tatsumi et al., have done laboratory studies investigating powdered, unbound Pt K L-zeolite catalysts for aromatization of hexane. They have looked at the effect of adding KCl and the addition method on catalyst performance [Chem. Lett., 387-390 (1993); J. Cat. 147, 311-321 (1994); Bull. Chem. Soc. Jpn., 67, 1553-1559 (1994); and Cat. Lett., 45 (1997) 107-112]. They have also investigated the effect of other halogen anions (F, Br, I) on hexane aromatization [Cat. Lett., 27 (1994) 289-295 and Sci. & Tech. in Cat., 117-122, (1994)]. They report that chloride and fluoride give the best results. Tatsumi et al., typically add a potassium halide (e.g., KCl) after Pt addition. In general, these authors add halide along with alkali, i.e., then add a refractory alkali halide salt to the catalyst. (Note: these catalysts are not hiz-cats, as defined herein.) In trying to understand why KCl addition improves catalyst selectivity, the authors of the J. Cat., 147 (1994) article add ammonium chloride to a powdered Pt L-zeolite catalyst that had been prepared by ion exchange (and washing). This catalyst, which is a hiz-cat, did not show the improved selectivity of catalysts with KCl added, leading the authors to discuss the importance of adding salts containing both alkali and halide.
Sugimoto et al., in Appl. Cat. A: General 96 (1993) 201-216, have also studied unbound catalysts. They prepared powdered hiz-cats from a variety of zeolites, including those prepared from an L-zeolite powder. These catalysts were made using a halocarbon treatment following either an alkaline soaking or ion exchange. In either case, the powdered zeolites were washed. Catalysts prepared from these washed powders were tested for n-hexane aromatization. As already noted, these studies were done on powdered catalysts, not bound catalysts.
None of the references discussed above address problems associated with commercial startup of bound halided zeolite catalysts. Nor do they teach the importance or desirability of washing bound zeolite bases or bound catalysts, especially non-acidic zeolite bases or catalysts, before adding halide as discussed hereinbelow.